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Later, on the homeward voyage, Worden conducted a more esoteric experiment. Pointing his instruments at X-ray sources far out in the galaxy, he recorded the emissions for clues that might be used to confirm the existence of "black holes"—weird, theorized remnants of huge, collapsed stars. He also awed earthlings—including his two daughters—by taking a televised 18-min. walk in deep space some 200,000 miles from earth to retrieve the exposed film cassettes from the service module's cameras.
Brief Concern. Two hours after their liftoff, Scott and Irwin were reunited with their hardworking buddy. After passing the precious cargo of moon rocks into Endeavour and closing the hatch, Scott said wistfully: "The Falcon is back on its roost and going to sleep." In fact, it came to a thunderous end. After a brief flurry of concern because of a possible hatch leak, the astronauts cut loose the lunar module's ascent stage and sent it crashing back to the moon's surface 59 miles west of Hadley Base. Its impact jiggled all three of the nuclear-powered seismometers on the moon, including the new Apollo 15 instrument. Geophysicist Gary Latham of Columbia University was delighted.
The shock waves, he reported, indicated that the moon has a crustlike surface layer at least 15 miles thick.
For the next two days, the reunited crew continued circling the moon—performing more experiments, photographing parts of the lunar surface never before seen by man, and in the case of Scott and Irwin, catching up on some badly needed sleep. On the last day in orbit, activities perked up. The astronauts were awakened by the theme from the film 2001: A Space Odyssey and later launched a 78.5-lb. scientific sub-satellite into orbit around the moon. Almost immediately, earthbound controllers detected radio signals from the $1.7 million instrument package. The satellite's sensors will provide new information about the plasmas and magnetic field in the vicinity of the moon. Ground trackers, recording irregularities in the satellite's orbit, will also be able to map the variations in the moon's gravitational field believed to be caused by "mascons" (for mass concentrations) under some lunar seas. As he caught a glimpse of the gleaming, spinning little moon outside his window, Scott exclaimed: "Tally ho! That's a very pretty satellite out there."
Good Burn. Just before Endeavour disappeared behind the moon on its 74th revolution, ground controllers gave it a "go" for one more important maneuver: firing the command ship's big 20,500-lb.-thrust engine to kick it out of lunar orbit. "Set your sails for home," said Mission Control. "We're predicting good weather, a strong tailwind, and we'll be waiting on the docks." Thirty-five minutes later, as Endeavour re-emerged and regained radio contact, Scott gave the happy word. The 2 min. 21 sec. burn had been flawless. Said Scott: "Endeavour is on the way home."